Reconfigurable multi-channel transmitter for dense wavelength division multiplexing (DWDM) optical communications

ABSTRACT

The invention provides a high performance reconfigurable DWDM transmitter incorporating low cost discrete optical components which can be placed in v grooves etched in a silicon optical micro-board or the like, keeping costs of manufacturing low. The lasers can be packaged in modules based on the technology of meso scale optics. The physical size of a multi channel module can be no bigger than a conventional single laser module.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. patentapplication Ser. No. 09/610,312 filed Jul. 5, 2000, which is anon-provisional patent application of and which claims the benefit ofpriority from U.S. Provisional Patent Application No. 60/212,431 asfiled Jun. 16, 2000, the full disclosures of which are incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] The field of the present invention relates in general to opticalcommunication using inexpensive, stable and accurate devices forcreating channels for wavelengths of light. More particularly, the fieldof the invention relates to a wavelength reconfigurable, multiplechannel transmitter for optical Dense Wavelength Division Multiplexedsignals.

BACKGROUND OF THE INVENTION

[0003] Optical fiber has nearly unlimited bandwidth, yielding 20 to 50times more bandwidth than copper cable. Optical fiber is now thesolution of choice at the wide area network (WAN) level. Implementationof fiber optic communication at the local area network (LAN) level willenable users to break current bottlenecks in the last mile ofinformation transfer. A further attraction of fiber-optic technology isits scalability. Most current fiber LAN products are Ethernet-based in arange of 100-Mbit/second up to 1-Gbit/s. Fiber optical communication iseasily scalable up to 10 Gbits/s, and several equipment vendors haveannounced fiber-optic links that can support transmission up to 1terabit/s utilizing more than 128 DWDM.

[0004] Optical telecommunications networks require significantlyincreased bandwidth to handle current and projected communicationstraffic. Most optical networks use time division multiplexing (TDM) witha single laser transmitter as a means of combining many separatetransmissions, allowing data rates of up to 10 Gbits/sec. The currentmarket trend is toward systems that use many individual transmitters,each of a different wavelength to increase channel capacity (an approachknown as wavelength division multiplexing, WDM). For example, atransmitter consisting of 8 distinct wavelengths provides 8× thecapacity of a single channel network (e.g. 80 Gbits/sec). Furthermore,WDM systems are scalable to 16, 32, 64, 128 etc. distinct channels, andmake the most efficient use of the extremely high bandwidth of opticalfiber communication networks. Therefore, what is needed is a system forproviding a stable, rapidly reconfigurable, multi-channel source for WDMand DWDM (dense-WDM) optical communications networks which can meetanticipated bandwidth demand.

[0005] A fiber optic communication link provides a virtually noise freemedium for transporting complex signals without distortion andinterference. Fiber optic cable has losses as low as 1-2 dB perkilometer, which is much lower than the 1-2 dB per 100 ft. for coaxialcable. Due to high frequency of laser light, fiber optic cables providea bandwidth in which many channels of information can be transportedacross a single fiber cable. The laser diodes' high efficiency, smallsize, high reliability, and low cost ($5-$10 for infrared, $40-$50 forvisible light) make them the ideal choice for communication devices.However, other components in these devices escalate the cost of presentoptical DWDM transmitter devices.

[0006] Low cost optical DWDM optical communication devices are notavailable in the market due to the current trend toward integratingcostly solid state semiconductor lasers and associated opticalstructures, and because optical solid state control structures are newand changing rapidly. The latest integrated optical components arecreated lithographically on a semiconductor chip substrate or on glass.However, it is very difficult and expensive to make multiple laserslithographically (in VLSI) which are locked to a reference grid.

[0007] Therefore, what is needed is a way to reduce the cost of buildingstate of the art DWDM optical communication devices. What is also neededis a method for shifting the burden of providing a high performance DWDMoptical communication system from costly precision optical components toinexpensive solid state control structures. It would be desirable toachieve a high performance DWDM transmitter using low cost opticalcomponents having a wider range of tolerances than is presently possible

[0008] Conventional WDM/DWDM sources lack stabilized frequencyreferencing and rapid reconfigurability in the event one or more lasersfail or are disrupted. Fabry-Perot interferometers (FPIs) have been usedto attempt to lock a laser to a stable frequency. Fabry-PerotInterferometers used as scanning interferometers can sense extremelysmall wavelength shifts when piezo-electric actuators (PZTs) are usedfor tuning the multi-pass dual mirror optical cavity. However, theseinterferometers require adequate wavelength references for long-termstability.

[0009] Molecular absorption line sources from various gases have beentried, in particular acetylene, and offer a potential medium for multiwavelength referencing. However, their unevenly spaced absorption linesand absorptive operation makes them difficult to tune for FPI-basedspectral applications. FPIs with narrowly—spaced uniform transmissionpeaks(˜100 GHz) have often been considered for an absolute frequencyreference when locked to atomic or molecular absorption lines. Glance,B. S. et al. (1988), “Densely Spaced FDM coherent Star Network withOptical Signals Confined to Equally Spaced Frequencies,” IEEE J.Lightwave Technol. LT-6:1770-1781. and others. While there have beensome applications for stabilizing laser arrays, such methods arerestricted in wavelength positioning and require active feedback controlin conjunction with costly precision optical structures, thereby drivingup costs.

[0010] Fiber Bragg gratings (FBGs) can produce a narrow band responsearound a single wavelength. However, their narrow response band and overwavelength span poses limitations. Some of the current alternatives foroptical transmitters provide only partial solutions. Examples of someconventional solutions and their disadvantages are described below. Mostdescribe an optical reference source. None use a gas spectral linereference source.

[0011] U.S. Pat. No. 5,892,582 discloses a fiber Bragg grating (FBG)source which provides spectral output at a selected wavelength within awavelength range. Thus, the FBG is used to provide a reference whereinthe spectral output of the FBG marks a peak of a comb identifying itswavelength. The FBG comb is used as the reference frequency source. Thisapproach is taken because atomic or molecular spectral lines are deemedunsuitable for the purpose of providing a stable reference source due tounevenly spaced absorption lines, and therefore are too restrictive inwavelength positioning. While this is useful in identifying andmeasuring wave-lengths of radiation from optical sources, it doesnothing to provide an inexpensive multi channel, reconfigurable opticaltransmitter.

[0012] U.S. Pat. No. 5,646,762 discloses another conventional approachto establishing a reference source comb using a voltage source connectedto a detector and tunable etalon comb of frequencies. A digitalprocessor connected to a photodetector and to the voltage sourcecontrols the tunable etalon. The digital processor further containsmemory for storing tuning voltages for wavelengths and for storingtuning voltages for temperatures. However, this provides a costlypartial approach limited to providing only a comb of uniformly spacedoptical channels from an already presumed stable input referencefrequency source. Furthermore, there is no provision for establishing astable reference frequency source for the etalon comb, or to providestability for multiple channels or reconfiguration of channels.

[0013] U.S. Pat. No. 5,949,580 discloses a controllable light amplitudedivider for dividing light at a particular wavelength into two portions,which together represent the amplitude of the input light. Thearrangement can be cascaded in a manner which operates as a multiplexeror demultiplexer. The main thrust of the '580 patent is to provide afast multiplexer and demultiplexer using an etalon. This arrangementrequires costly precision optical components and is not suitable for ahigh performance reconfigurable DWDM application. Furthermore, the '580patent is not a solution for an optical transmitter, which has manyother functions, and the multiplexer/demultiplexer scheme is notnecessary for a most optical transmitters

[0014] U.S. Pat. No. 4,813,756 discloses a device for interconnecting orfor linking two optical fibers comprising a mechanically rotatableetalon arrangement. The '756 patent principally creates a device forinterconnecting two optical fibers. While multiple etalons are used invarious configurations, this does not provide a full multichanneloptical transmitter, but rather a partial and very expensive way toconnect DWDM fiber. '756 also claims optical channel selection filtermounted for coupling two single mode optical fibers.

[0015] U.S. Pat. No. 4,707,061 discloses an optical communicationssystem using a resonant cavity for supporting a set of resonant modesand introducing predetermined reference wavelengths. A means forcontrolling the resonant cavity tunes one resonant mode to thewavelength of a fixed wavelength light source. Semiconductor lasersources are used in conjunction with resonant cavities which must bepresent at the transmitter as well as the receiver, thereby addingexpense and complexity to an all ready cumbersome scheme to communicateover an optic network.

[0016] U.S. Pat. No. 5,673,129 discloses a plurality of opticaltransmitters for outputting optical signals, at least one opticalwavelength selector communicating with the optical transmission. Thewavelength selector includes a Bragg grating member with a wavelengthband of high reflectivity. The wavelength band of high reflectivity foreach Bragg grating member corresponds to an optical channel output. The'129 patent provides a closed loop optical system and uses semiconductorlasers, thereby resulting in a costly approach and complex system usinggain bands, amplifier stages and pumps.

[0017] U.S. Pat. No. 6,014,237 discloses a multi wavelength mode-locked(MWML) laser source including a semiconductor optical amplifier (SOA)disposed in a cavity of the MWML laser source. The SOA is activelydriven by a radio frequency (RF) signal and emits periodic pulses withina plurality of discrete wavelength bands simultaneously. Thesemiconductor optical amplifier and radio frequency (RF) signal drivermake this a relatively expensive solution. The '237 patent also requiresthat input signals be multiplexed by a high speed electronic time domainmultiplexer (ETDM) to a higher bit-rate electronic data stream forcoding by an optical modulator in the optical pulse stream emitted bythe MWML-DWDM. This results in complex design and interfacingrequirements which are not suited to a practical, low costimplementation.

[0018] U.S. Pat. No. 6,044,189 discloses a temperature compensatingfiber Bragg grating contained in an optical fiber. The '189 patent isconcerned with the improvement of control of a fiber Bragg grating, onlyone possible component of a DWDM system.

[0019] U.S. Pat. No. 6,028,881 discloses a pump source tunable among aplurality of pumping wavelengths; a plurality of waveguide lasersresponsive to respective pumping wavelengths for emitting light. The'881 patent introduces a method of combining solid state waveguidelasers with semiconductor lasers and enhancing electrical tunability.Components include intra-cavity pumping and pump reflectors. Theserequire expensive integrated optics to manufacture.

[0020] U.S. Pat. No. 5,953,139 discloses an analog light wavecommunication system having at least two optical transmitters. The firstWDM receives optical information signals from the optical transmittersand multiplexes the optical information signals to a composite opticalsignal at an output. Each input of the WDM comprises at least oneoptical resonant cavity; an oscillator circuit providing a single tonemodulation signal and a phase modulator having an optical input coupledto the output of the WDM. The single tone modulation signal drives acomposite optical signal which is too restrictive for most opticaltransmitter uses.

[0021] Conventional WDM/DWDM sources fail to provide the stabilizedspectral frequency referencing or the rapid optical channelreconfigurability necessary for a high performance, low cost opticalDWDM system. The design of conventional optical transmitters is directedtoward the use of integrated optics wherein all optical components arecreated lithographically on a semiconductor chip substrate or on glass.These can be difficult and expensive to manufacture, as multiple lasersmust be locked to a reference grid lithographically, using VLSItechniques.

[0022] Conventional DWDM approaches are not cost-competitive in themetro markets where they must compete with lower cost all electronicproducts which are more dynamic and operate at lower bandwidth demands.Most conventional DWDM systems operate at OC-48. As demand for higherbandwidth increases, these data rate demands will migrate to Metromarkets. Therefore, what is needed are low cost optical devices whichare dynamic from a standpoint of configurability and can meet higherbandwidth demands.

[0023] DWDM optical devices demand very large bandwidths. A failure ofone optical channel affects multiple protocol stack layers and thuslarge numbers of users. Therefore, in the event of a failure at thechannel level, restoration must occur at multiple stack levels. Thespeed of restoration is critical. Bandwidth reservation is an option forthese slower layers, but this requires that excess idle capacity must bebuilt in at the optical device level.

[0024] In the protocol layer scheme for optical channels, the InternetProtocol (IP) layer is carried by the ATM layer below. IP over DWDMpresents topology node architecture issues. There is a virtual mappingbetween the physical and logical topology of IP over ATM, which leads toscaling challenges. One of the solutions is to make every switch into arouter. The current cost of optical routers makes this a very expensivesolution. What is needed are inexpensive optical switches and routers.

[0025] In high reliability networks, reconfigurability options arenecessary. However, conventional designs are very expensive. Forexample, each channel has a fixed-frequency laser, in addition to thelaser that carries the data, the “active” laser. The fixed frequencylaser is a spare, identical to the active laser that initially carriesthe modulated signal. The spare laser must be prepared to carry themodulated signal on the active laser's specific carrier frequency. Bothlasers are generally physically located in the same rack.

[0026] Thus, in the event of a laser failure, the spare fixed-frequencylaser takes functional control on only the designated frequency andsignal. However, this approach forces the manufacturer to fabricate aredundant number of lasers, at least one spare for each active laser, toensure a high reliability device. Therefore, what is needed is anoptical channel device which requires fewer lasers, lower productioncost and provides high reliability. What is also needed is a DWDMtransmitter which is reconfigurable, not only with respect to lasers,but also across channels.

BRIEF SUMMARY OF THE INVENTION

[0027] In order to overcome the foregoing deficiencies in conventionaloptical DWDM systems, an aspect of the invention provides a highperformance reconfigurable DWDM transmitter incorporating low costdiscrete optical components which can be placed in v grooves etched in asilicon optical micro-board or the like, keeping costs of manufacturinglow. The lasers are packaged in modules based on the technology of mesoscale optics. The physical size of a multi channel module is no biggerthan a conventional single laser module.

[0028] In another aspect of the invention, direct wavelength monitoringis achieved by using a wavelength modulation locking technique appliedindependently to a gas absorption line and to etalon fringes. Thefrequency stability and resolution achieved thereby make it possible topack channels closely and achieve spacing up to the modulation limit,filling the available bandwidth. This now enables high density DWDM topopulate as many channels as desired to the modulation limit.

[0029] An aspect of the invention uses a set of n lasers and k sparesources, wherein each laser is actively locked to a set of equallyspaced wavelengths according to the ITU frequency grid, andsimultaneously to a stable spectral reference wavelength. The set ofequally spaced frequencies is generated by an etalon, acting as afrequency comb generator. The absolute wavelength standard is providedby a gas absorption cell. The wavelength of each channel can be changedon a millisecond (msec) time scale under microprocessor control in theevent that any channel should fail, thereby enabling substantiallyinstantaneous reconfigurability.

[0030] According to this aspect of the invention, a separate (fixedfrequency) spare laser is not needed for each active (fixed frequency)laser. The invention enables a single laser to be used as a substitutefor a number of fixed-frequency lasers, and a number of fixed-frequencyspare lasers as well.

[0031] In addition, any “active” laser (i.e. a laser already assigned toa particular channel, not a spare) can be reassigned to a differentchannel, if necessary, which further improves network reliability. Ifall spare lasers within a module should fail, and an active laser at themost valuable channel fails as well, the system still can carry thetraffic over the most valuable channel by reassigning a laser from oneof a lesser used or less valuable channels to the most valuable channel,until physical replacement of lasers is made. The modulation signal isswitched electronically to modulate the spare laser instead of the laserthat failed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] These and other features and advantages of the present inventionwill become more apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

[0033]FIG. 1 is a diagram showing basic components of a frequencystabilized laser for one channel according to an aspect of theinvention.

[0034]FIG. 2 is a schematic diagram showing a multi-channelre-configurable DWDM transmitter according to an aspect of theinvention.

[0035]FIG. 3 is a diagram showing a simple configuration of amulti-channel reconfigurable transmitter according to an aspect of theinvention.

[0036]FIG. 4 is a graphical display illustrating the output produced bya multi-channel reconfigurable transmitter according to an aspect of theinvention.

[0037]FIG. 5 is a block diagram illustrating operative connection ofcomponents according to an aspect of the invention.

[0038]FIG. 6 is a drawing showing an example implementation of a lowcost multichannel re-configurable transmitter according to an aspect ofthe invention.

[0039]FIG. 7 is a flow chart of the control logic which enablesreconfigurability according to an aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0040]FIG. 1 shows the basic components of a frequency stabilized diodelaser wherein stabilization occurs through a spectral line. An overviewof a simple mode of operation for one channel starts with a laser source102, in this case a tunable diode laser. The laser light is directedinto a collimating lens 106 and into a gas absorption cell 108,comprising a chamber holding gas of known spectral line characteristics.This gas is used to lock onto a reference frequency (wavelength) usingwavelength modulation spectroscopy (WMS), also known as derivativespectroscopy. A beam splitter 110 can be used to forward a part of thebeam through a lens 131 and to a photodetector 132 which enables thelaser frequency to be compared and tuned to a spectral line frequency ofthe known gas in the reference gas cell 108.

[0041] This part of the process is designed to be a self-tuningprocedure: the diode laser 102 is turned on, and locked to a gasabsorption line from gas absorption cell 108 using a standard WMStechnique. Detector 132 is used to monitor light transmission throughthe cell 108, and a reference frequency control loop 528 (FIG. 5)includes a microprocessor (530 shown in FIG. 5) for feedback in tuninglaser 102 to the reference frequency. The gas absorption cell 108contains a suitable gas, for example acetylene, which has numerouswell-known narrow absorption lines in the 1550 m optical communicationswindow. Other gases may be used for other windows as needed. Wavelengthmodulation spectroscopy is implemented with diode lasers having theadvantages of small size, non-intrusiveness, speed, and ease of use.

[0042] Next, the reference frequency stabilized diode laser is frequencylocked to a sequential fringe frequency of a (temperature) tunablestable Fabry-Perot etalon 112. The beam proceeds through to the etalon112 which is tuned to a position where an etalon transmission fringecoincides with the laser wavelength locked to the reference spectralline derived from gas absorption cell 108. Passing through a lens 113and into Detector 2 114, the transmission is monitored for tuning. Thefree-spectral range of the tunable etalon 112 is chosen to be equal tothe required ITU grid spacing (e.g. 100 GHz for the current ITUstandard), thus the etalon 112 transmission spectrum consists of aseries of regularly spaced transmission peaks. Each of thesetransmission peaks will constitute a channel when fully initialize,tuned and configured.

[0043]FIG. 2 is a block diagram showing a multi-channel re-configurableDWDM transmitter. The channel shown above in FIG. 1 is just one of nchannels in a bank. Referring to FIG. 2, one common gas cell 208comprising a gas of a known spectral line frequency and one etalon, alsocalled the grid generator 212 establish a stable frequency grid for aplurality (n) number of independent active lasers 202 and therefore (n)channels. All lasers 202 have the same reference data structures sincethey all lapse through the same gas cell 208 and are locked ontosuccessive fringes produced by common etalon 212. The referencefrequency control loop 228 enables initialization and tuning of eachlaser in the bank to the reference spectral frequency derived from gascell 208 in a well known manner. The grid positioning control loop 226sequentially tunes each independent active laser 202 in the bank to oneof the fringe frequencies of grid generator 212 using the gridpositioning control loop 226 for stabilizing and controlling the fringecount and fringe frequency tuning.

[0044] Direct monitoring of output wavelength within grid positioningcontrol loop 226 and reference frequency control loop 228 is used tostabilize lasers 202. Direct wavelength monitoring is achieved by usingwell known laser spectroscopy wavelength modulation locking techniquesapplied independently to a gas absorption line derived from gas cell 208and to etalon fringes derived from the grid generator. This is farsuperior to using “blind” control loops to stabilize a laser'stemperature and current 212 as is done in many conventional DWDMsystems. Both control loops 226 and 228 are microprocessor controlledand provide two essential stages in the system initialization to lockand maintain lasers at stable frequencies. Thus, the initializationprovides self-stabilizing and self-calibrating optical channels.

[0045] These two processes are under the control of two independentfrequency control loops, the first one tunes the laser and controls thelaser injection current; the second one tunes the etalon by controllingits temperature. The two control loops could be merged into the sameelectronic digital control circuit as well. The two independent controlloops 226, 228 also enable independent control of a plurality (n) lasers202 to be connected in parallel in a bank. This makes possible aconfigurable feature of the optical transmitter, which is covered indetail below.

[0046] Reference positions are now firmly defined and each laser 202 inthe bank is locked to a fringe frequency. These dense wavelengths (referto FIG. 4) are then multiplexed into the optical communication fiber224. That is, the fiber optic cable is physically hardwired to theoutput of the transmitter module. This is an advantage inreconfiguration in that the optical paths of alternate or spare lasersare always connected to the output of transmitter module 225. Theoptical fiber output carries different wavelength light 232 at uniformchannel spacing 230.

[0047]FIG. 3 is a diagram showing another configuration of amulti-channel reconfigurable transmitter according to an aspect of theinvention. Referring to FIG. 3, a plurality (n) of laser diodes 302 areconnected in parallel in a bank. Although many types of laser sourcescan be used, the lasers are preferably Distributed Bragg Reflector (DBR)lasers with a characteristic tunability of 10-30 nanometers in a singlemode output or narrow band output. Such lasers are typically low costand tunable over a wide range in comparison to other lasers. DBR lasersoffer the widest wavelength coverage. Distributed Feedback (DFB) lasersare acceptable as an alternative if each laser is required to a smallwavelength range, i.e., a small number of channels. The laser beams arecollimated in corresponding GRIN lenses 306 and directed into the gasabsorption cell 308. A common gas absorption cell 308 provides amolecular absorption line stabilized to a molecular transition, which isthe reference frequency for all lasers. The reference stabilized beamsthen pass to a first set of photodetectors 332 which enable frequenciesto be mapped to voltages which provide the feedback loop means fortuning the lasers to the stabilized molecular absorption line. Uponinitialization, the lasers 302 become locked to an absolute referencefrequency.

[0048] The stable reference tuned lasers are further directed into theetalon 312 and on to the second set of detectors 314, where they areeach tuned to an etalon comb frequency fringe. The etalon 312 is a solidetalon comprising a glass plate with coated surfaces and is temperaturetunable, as is well known. The output transmission spectrum from theetalon 312, that is, the distance between peaks and valleys in thecorresponding wave form produced by the etalon, depends on the index ofrefraction, temperature and thickness of the etalon. The detectors 314and 332 are photo diodes having a spectral range of (for example) 155nanometers and are gallium arsenide photo diodes. However, other laserswith suitable characteristics may be used. Examples are: diode lasers,gas lasers, chemical lasers, masers, or the like. The feedback loops areshown in FIG. 2. The bank of tuned laser diodes 302 and theircorresponding channels are optically coupled to the fiber optictransmission line 318: The etalon 312 (e.g. 10-100 GHz free spectralrange) serves as the generator of an optical frequency comb with e.g.10-100 GHz separation. Each of the lasers 302 is tuned to one of thefringes produced by the etalon 312, and the etalon grid is locked thereby its respective frequency control loop (228 in FIG. 2).

[0049] Each laser 302 is passively stabilized so that, at the startingpoint, it finds itself within the capture range of the loop, centeredaround a reference wavelength λ₀. Each laser 302 is then brought to awavelength λ_(n) by either temperature or injection current tuning, orboth. Starting from the reference wavelength λ₀, each laser 302 is inthe next step brought to any position on the etalon grid (400 in FIG. 4)produced by etalon 312 by tuning its wavelength in the selecteddirection (direction from the reference frequency fringe to a selectedfringe), and monitoring the laser output as it passes through etalonfringes produced by etalon 312. The position of the laser's frequency onthe etalon grid 400 is determined by counting the fringes. In this way,each laser 302 can be tuned to any of n wavelengths (“channels”). Thechannels are independently modulated (data impressed on the lasercarrier) with either an external or an internal (electro-absorption)modulator, integrated in the diode laser chip.

[0050] The device operates and initially tunes itself under standardmicroprocessor control techniques which are well known, which is simpleand inexpensive. The entire system operates under the microprocessorcontrol, which performs the auto-calibration procedure described above.In this way, an aspect of the invention provides a multi-wavelengthsource for DWDM that is self-calibrating, self stabilizing and selftunable. This aspect of the invention provides an advantage overconventional systems in that if one of the lasers should fail, resultingin a temporary channel loss, an alternate laser can be tuned to thedesired wavelength.

[0051]FIG. 4 is a graphical representation of a wave form illustratingthe output produced by a multi-channel re-configurable transmitter. Thetransmission spectrum is shown as optical power 401 as a function ofwavelength 408. The reference source wavelength 404 provides an anchorfor a uniform spacing output grid, such as ITU grid 410. (ITU grid 410is an etalon grid with a specific channel spacing.) For example, thegrid 410 consists of frequencies with channel spacings 402 which complywith ITU standards.

[0052] An aspect of the invention is shown in FIG. 2 wherein lasertuning is accomplished using two independent control loops, the firstcontrol loop capable of locking an entire comb. The second loop locksany laser in a plurality of lasers to a given point on the comb. Thisthereby provides superior frequency stability and resolution. Thefrequency stability and resolution achieved with this aspect of theinvention makes it possible to pack the channels closely and achievesstability and spacing reduction possible down to the modulation limit,thereby filling the available bandwidth. This now enables high densityDWDM to populate as many channels as desired to the modulation limit.

[0053] The ability to populate as many channels as desired up to themodulation frequency may provide enabling technology for a low costswitchless network; in particular, a high density metropolitanswitchless network.

[0054]FIG. 5 is a block diagram illustrating the operation of thecontrol of the laser path through its optical components. It will beappreciated that the optical path is decoupled from its electroniccontrol elements. This decoupling enables production of a highperformance, fully reconfigurable DWDM transmitter through the use ofinexpensive electronic components. It shifts the burden of providinghigh density DWDM from costly precision optical components integrated inVLSI to inexpensive electronic control circuitry. In FIG. 5, the controlelements and circuits are depicted in broken lines and the optical pathsare drawn in solid lines.

[0055] In operation, when laser 502 is turned on, first detector 532looks at output using standard wavelength modulation spectroscopy. Laser502 is tuned under microprocessor control in a known manner over awavelength range in which it finds the absorption line of the gas in thegas absorption cell. Some of the beam can be diverted through the use ofa beam splitter 510 and directed to a first detector 532.

[0056] First detector 532 provides control information to the referencefrequency control loop which is applied in real time to keep the laser502 wavelength locked to the absorption line. To accomplish this, firstdetector 532 measures the intensity of the beam, detects the peak ofabsorption line, a control mechanism provides information on laserwavelength coincident with the absorption line, determines whether laserwavelength is coincident with the absorption line or not then makes acorrection signal to 502 corresponding laser.

[0057] Correction is applied so the first detector 532 always seeswhether the laser wavelength is coincident with gas absorption line.Once the microprocessor/controller 530 recognizes the laser wavelengthis stable and locked to a reference source 508,microprocessor/controller 530 tunes the etalon 512 by changing thetemperature and monitoring the output of second detector 514. Thedetector 514 is maximum amplitude seeking and detects when a local peakor comb tooth wavelength is found. The microprocessor 530 receives thesignal from the second detector 514. Etalon 512 has a transmissionspectrum consisting of a fringe or comb. As microprocessor/controller530 tunes the etalon, it moves the comb in wavelength space and makesone fringe of the comb coincident with laser wavelength. Since thereference frequency is known from the reference frequency control loop528 and coincident with one of the comb frequencies,microprocessor/controller 530 can set any laser frequency by countingthe fringes while changing the wavelength of the tunable laser 502 bychanging the injection current to the laser 502 or the etalontemperature 532 or both.

[0058] In this fashion, microprocessor/controller 530 can tune any ofthe lasers 502 to any of the channel frequencies 406. For example, totune laser 502 to fringe 406, microprocessor/controller 530 addresseslaser 502 by first tuning to absorption line. Once the reference pointis known on the etalon grid 512, microprocessor/controller 530 countsthe fringes while changing the wavelength of the laser by changing theinjection current or the etalon temperature or both. The second detector514 provides a correction signal to the etalon stabilization loop 526which locks the tunable etalon to the wavelength of the laser wavelengthdesired. By reading the output of the second detector 514,microprocessor/controller 530 can use this information to make light togo through tunable etalon fringes and lock on to any desired frequency.

[0059] The foregoing process establishes a reference grid with stabledefined wavelengths at any point, which can be described as a referencewavelength+/−integer number of free spectral range spaces. This equalsdistance between fringes or channel spacing and is shown in FIG. 4.

[0060] The tuned laser is directed into the output coupling 518 byoptical fiber connection which is then multiplexed into the opticalfiber transmitter output 522. The optical path is preserved by havingall lasers multiplexed into the communication fiber 522 at all times.That is, individual channels 518 are physically hardwired (opticallyconnected) to the output fiber 522. All lasers, even spares oralternates 504 not currently active, are connected through the use ofcouplers, to the output of the transmitter multiplexer 522 module, andthe lasers go online “active” as needed during reconfiguration. Thus, anaspect of the invention provides the capability to reconfigureindividual tunable lasers to maintain optical network integrity.

[0061] Each of the lasers 502 is tunable over any number of channels(e.g. 10-20). Therefore, lasers 502 are wavelength selectable, as wellas wavelength re-configurable. Since each laser 502 in the bank can betuned to work on a multiplicity of wavelengths or “channels” and we alsohave a dynamically selectable wavelength multi-channel transmitter.

[0062] The microprocessor/controller 530 manages a lookup table. Thelook up table is produced during the initial calibration process andstored in the memory of a controller 530. The parameters in the lookuptable include individual laser operating points, and parameters, such asinjection currents, Bragg reflector control currents, substratetemperatures and individual wavelength modulation signal outputaddresses, comb fringe count and other data needed to reconfigure (undersoftware control) laser operation in the event of a channel failure.Replacing a laser that has failed without manual intervention bymicroprocessor/controller 530 which incorporates a look up tablecomprising operational parameters of all currently active lasers. When afailure is sensed, microprocessor/controller 530 elects an alternatelaser, initializes the new laser to the failed channel specifications,attaches the failed laser's modulated signal to the new laser channeland continues operation within 20-50 milliseconds or less. Thisoperation is seamless and transparent to a user due to the tolerances incommunication protocol.

[0063] Upon an active laser 502 failure, the detector 514 senses thefailure of the down fringe, and notifies the microprocessor/controller530 of the failed comb number. Other convenient ways of sensing laserfailure are well known and also may be employed. Themicroprocessor/controller 530 initializes an alternate laser 504 to theproperties of the failed laser 502 found in the lookup table. The spare504 is tuned to the wavelength of the failed channel 502 and themodulated signal is vectored to the spare 504 laser for transport.

[0064] The foregoing aspects of the invention provides a reconfigurabletransmitter, whereby any tunable spare laser can be brought on linesubstantially instantaneously and automatically, upon an active laserfailure, regardless of the failed laser's carrier wavelength. Inaddition, if the spare fails immediately or before another active laserfails, another spare can be brought on line to substitute for the spare,thus providing an additional back up. This chain of redundancy canprogress to exhaust all spares, thereby providing an extremely highreliability and reconfigurable device, which is not possible withconventional DWDM transmitters. With the exhaustion of all spares,active channel lasers can be substituted for the highest demand or loadcarrying channels to continue the operation. This is not currentlypossible with the optical transmitters. This aspect of the inventionprovides a method requiring far fewer spare lasers, while stillincreasing transmitter and therefore network reliability, simply byvirtue of the reconfigurability for any number of fixed-frequencylasers.

[0065]FIG. 6 is a drawing of a low cost implementation of multi-channelre-configurable transmitter according to an aspect. of the inventionThis aspect of the invention provides a modular, meso scale approachwhich decouples the performance of an optical DWDM transmitter from theperformance of its optical components. It eliminates the need for costlylithographic integration of optical components, while at the same timeincreasing channel capacity to the modulation limit as set forth above.

[0066]FIG. 6 shows a low cost meso scale implementation which usesdiscrete optical elements mounted on v grooves etched in a silicon 642optical micro-board. Laser diode 602 is disposed between lens 606 andlens 620 as shown. Gas cell 608 and etalon plate 614 are all discrete,components mounted in the v grooves 642. This is in direct contrast toconventional integrated optics, wherein all optical components arecreated lithographically on a semiconductor chip substrate or on glass.Discrete optical components according to an aspect of the inventionenable system to be modularized.

[0067] This embodiment eliminates the need for difficult to manufactureand expensive to make multiple lasers which are locked to a referencegrid lithographically, using VLSI techniques. An aspect of the inventionprovides a solution by using low cost discrete optical components whichcan be placed in v grooves etched in silicon optical micro board. Lasers602 are packaged in modules based on the technology of meso scaleoptics. The physical size of a multi channel module is not bigger thanthe current single laser module, on the order of millimeters.

[0068] Referring to FIG. 6, a DBR laser 602 is mounted on a siliconoptical microboard 642. A DBR laser is low cost relative to a fixedfrequency distributed feedback laser DFB, currently used. The entireassembly is fixed to a ceramic base 640 or other suitable substrate.

[0069] The individual signal input circuitry converts the digital signalto the modulated analog signal via D/A converter. The intensity of theDBR laser 602 diode is modulated by adding AC current to the laserdiodes' bias current. Many other means to do this are possible and wellknown to those skilled in the art. This still enables precise and simplecontrol of the quiescent current. Bias current is necessary to keep thelaser diode operating in a region where the relationship between the ACcurrent and the change in intensity of the output is semi-linear.

[0070] The optical section transmits the signal from the lasers to thedetectors through a Gradient Index (GRIN) 606. The GRIN lenses 606 and620 are mounted in the grooves 642 on a silicon optical micro board, orother suitable substrate for mounting modular optical components.

[0071] A GRIN 620 is also used to introduce the laser light into thefiber on the output end. The light propagates in the fiber where signalsmultiplexed onto the output transmitter fiber. Since the beam of a diodelaser diverges severely from its source, a lens must be used to focusthe light onto the fiber 622. To minimize the loss between the photodiode and the fiber a GRIN lens is used. GRIN lenses 620 collimate abeam with minimal loss and aberration because their index of refractionvaries radially due to dopants infused into the glass of the lens. Byusing a GRIN lens with an uneven fractional pitch length, the photolight can be focused on the tip of the fiber.

[0072] The light then travels from the GRIN 606 through the gasabsorption cell 608 and impinging on the photodiode 632. The signal froma photodiode 632 (in the absorption case) is fed to the referencecontrol loop for tuning the laser to the gas spectral line frequency.This signal is used in real time to lock the light to the referencefrequency. This allows the remaining light traveling through the etalon612 impinging on the second detector 614 to have a base referencefrequency from which to tune to a comb frequency. The signal from thephotodiode 614 is fed to the etalon grid control loop for tuning to anindividual comb frequency.

[0073] In another aspect of the invention, by using active control, highperformance DWDM is achieved with low cost optical components of muchlooser specifications than is presently possible. In other words, thisaspect of the invention shifts the burden of maintaining highperformance from expensive optical components to inexpensive electroniccontrol components.

[0074]FIG. 7 shows the control logic steps in the reconfiguring of afailed optical channel. A laser failure signal starts thereconfiguration processes 702. The network has 20-50 milliseconds toactivate another laser to take over the functioning of the failed laserfor this to be seamless to light router protocol.

[0075] For example, laser no. 3 operates at 1552 nm and drops out,thereby producing a failure signal 702. The micro controller now must goto a spare or alternate laser and obtain all characteristic parametersfrom the incorporated lookup table. The lookup table contains all theparameters for the failed laser 705 which are to be used in the sparelaser.

[0076] All the necessary parameters for the failed channel are stored inthe controller lookup tables 705. These parameters contain, among otherdata, individual laser operating points, injection currents, Braggreflector control currents, substrate temperatures and individualwavelength modulation signal output addresses, comb fringe count.

[0077] The lookup table provides the injection current and the specificsubstrate temperature. The micro controller then selects an availablespare laser or less loaded alternate laser 708. There are typically twospare lasers for every eight active lasers. However, any convenientratio of spares to active lasers may be employed. To tune thespare/alternate laser to a reference spectral line frequency 711, themicro controller applies an injection current of about 35 mA to theactive region and any correction, as the laser control loop is openwhile tuning.

[0078] Next, the micro controller tunes the spare/alternate laser tofringe channel n of failed laser m 714. There are several ways to dothis and a combination may be the best. Micro controller counts thefringes while changing the wavelength of the laser by changing theinjection current or the temperature or both. The micro controllerincreases the injection current on the selected alternate/spare laserand monitors the second detector to see if signal of the selected laseris coincident with fringe count of the failed laser. The microcontroller keeps count of the fringes as it increments to match thelookup table fringe number of the failed laser.

[0079] Another method may simply use the Bragg reflector control currentfrom the lookup table to tune to the desired fringe, for example, 25 mato the Bragg region. When the micro controller has tuned the spare tothe failed laser fringe frequency, it has established a carrier for thechannel.

[0080] The modulated signal must then be applied to the spare lasercarrier channel. This can also be done in several ways. A simple way isto keep the modulated signal output circuit address in the lookup tablefor each laser and transfer this information to the spare laser uponlaser failure. The laser modulation signal is switched electronically tomodulate the spare laser instead of the laser that failed. Themodulation may be direct, or by means of a built in electro absorptionmodulator. If the modulator is an external device, nothing needs to beswitched, the external modulator does not care where the carrier lightcomes from.

[0081] As is well known, quality control is implemented to insure thatthe reconfigured channel is indeed carrying the interrupted signal 718.If the modulated signal is back online, a have a valid reconfigurationhas been established. The foregoing steps are then repeated with analternate laser until the micro controller reestablishes a reconfiguredchannel.

[0082] Conventional high speed Internet Protocol (IP) routing ischallenged by greatly increasing volumes of traffic. The capability tore-configure channels rapidly and automatically in accordance with theforegoing aspects of the invention, provides a basis for theaccommodation of advanced routing protocols such as resource reservationRSVP or multicasting. MPLS (Multiple Protocol Lambda Switching) overDWDM depends heavily on restoration through redundancy. Thus, theforegoing aspects of the invention could be applied to provide improvedMPLS.

[0083] The frequency stability and resolution achieved by the foregoingaspects of the invention make it possible to pack communication channelsclosely and maintain stability and spacing up to the modulation limit,thereby filling the available bandwidth. This would enable a highdensity DWDM system to populate as many channels as desired up to themodulation limit, and may support a switchless optical network or enablethe implementation of fault tolerant optical routers.

[0084] While this invention has been described in connection with whatare presently considered to be the most practical and preferredembodiments, it is to be understood that the invention is not limited tothe disclosed embodiments, but on the contrary, it is intended to covervarious modifications and equivalent arrangements which are includedwithin the spirit and scope of the appended claims.

What is claimed is:
 1. An optical DWDM transmitter device comprising: aplurality of lasers, each laser providing an optical channel; aplurality of active control loops locking each of the plurality oflasers to an associated frequency so as to enable an independentmodulation signal to be carried on each of the optical channels; and atleast one spare laser for transmitting on any of the optical channelswhen the associated laser fails.
 2. A device as in claim 1 wherein saidlasers comprise diode lasers, gas lasers, chemical lasers, or masers. 3.A device as in claim 1, further comprising a frequency comb including aFabry-Perot etalon, a spacing between the optional channels beingdefined by the etalon.
 4. A device as in claim 3, further comprising astable reference frequency defined by a gas absorption cell, the etalonbeing frequency adjusted in response to the reference frequency.
 5. Adevice as in claim 1, wherein the feedback loops comprise amicroprocessor/controller.
 6. A device as in claim 1 wherein the opticalchannels have frequencies separated according to fiber optictelecommunication standards.
 7. A device as in claim 1, wherein thespare laser takes over the functioning of the replaced laser,transmitting the replaced laser's frequency on a millisecond time scaleso as to enable restoration of DWDM optical layer communication withlittle disruption to transmission of any modulated signals.